Ion sources are used in implantation, sputter deposition, ion beam assisted deposition, ion spectroscopy, and direct ion beam deposition. In most conventional ion beam sources, ion beams are produced by extracting charged particles from a gas discharge (including plasma and arc-derived discharges).
One type of ion source is a contact or surface ionization source. A conventional contact ionization source for cesium ions is shown in G. R. Brewer, "Ion Propulsion: Technology and Applications", (Gordon and Breach, 1970), pp. 102-105. and includes a porous tungsten contact ionizer. Cesium is vaporized in a cesium reservoir and is transported to the porous contact ionizer through a manifold. The contact ionizer is kept at 900.degree. C.-1200.degree. C. Cesium ions are produced on the surface of the contact ionizer by surface ionization effects.
Examples of a thermionic emission solid state ion source are described by O. Heinz and R. T. Reaves in "Lithium Ion Emitter for Low Energy Beam Experiments," Rev. Sci. Instr., vol. 39, pp. 1229-1230 (August 1968) and by D. W. Hughes, R. K. Fenney and D. N. Hill in "Aluminosilicate-Composite Type Ion Source of Alkali Ions," Rev. Sci Instr., vol. 51, pp 1471-1472 (November 1980). Thermionic sources use aluminosilicate base alkali ion emitting compounds.
A very similar prior art system is described by M. Seidl in "Solid-State Source of Ions and Atoms", U.S. Pat. No. 4,783,595. The Seidl ion source combines the advantages of porous metal contact ionizers with those of aluminosilicate emitters. A porous refractory thin film is coated on the emitting surface of a solid electrolyte. Cesium is supplied to the emitting surface under the influence of a bias voltage applied across the electrolyte. Ion emission of cesium takes place on the surface of the porous tungsten thin film by surface ionization. Seong I. Kim and Milos Seidl describe the aforementioned solid state cesium ion source in "Cesium Ion Transport Across A Solid Electrolyte-Porous Tungsten Interface", J. Vac. Sci. Technol. A7(3), pp. 1806-1809 (May/June 1989) and in "A New Solid-State Cesium Ion Source", J. Appl. Phys., 67(6), pp. 2704-2710 (March 1990).
A solid state ion source has many benefits when compared with a gas ion source. A solid state ion source can be operated in &lt;10.sup.-10 Torr without the use of differential pumping or associated hardware necessary for operation of a gas ion source. The solid state ion source is compact and easy to operate.